doc: explain how to use functional indexes in migrations (#4149)

examples/functionalidx/example_test.go

@@ -0,0 +1,60 @@
+// Copyright 2019-present Facebook Inc. All rights reserved.
+// This source code is licensed under the Apache 2.0 license found
+// in the LICENSE file in the root directory of this source tree.
+
+package main
+
+import (
+	"context"
+	"errors"
+	"log"
+	"os"
+	"testing"
+
+	"entgo.io/ent/dialect"
+	"entgo.io/ent/examples/functionalidx/ent"
+	"entgo.io/ent/examples/functionalidx/ent/user"
+
+	"ariga.io/atlas-go-sdk/atlasexec"
+	"github.com/lib/pq"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTriggersTypes(t *testing.T) {
+	if os.Getenv("CI") != "" {
+		t.Skip()
+	}
+	ctx := context.Background()
+	client, err := ent.Open(dialect.Postgres, os.Getenv("DB_URL"))
+	if err != nil {
+		log.Fatalln(err)
+	}
+	ac, err := atlasexec.NewClient(".", "atlas")
+	if err != nil {
+		log.Fatalf("failed to initialize client: %v", err)
+	}
+	// Automatically update the database with the desired schema.
+	// Another option, is to use 'migrate apply' or 'schema apply' manually.
+	_, err = ac.SchemaApply(ctx, &atlasexec.SchemaApplyParams{
+		// URL to your database. For example:
+		// postgres://postgres:pass@localhost:5432/database?search_path=public&sslmode=disable
+		URL: os.Getenv("DB_URL"),
+		Env: "local",
+	})
+	require.NoError(t, err)
+
+	// Clean up tables before running assertion.
+	client.User.Delete().ExecX(ctx)
+
+	// Test that the unique index is enforced.
+	client.User.Create().SetName("Ariel").SaveX(ctx)
+	err = client.User.Create().SetName("ariel").Exec(ctx)
+	require.EqualError(t, err, `ent: constraint failed: pq: duplicate key value violates unique constraint "unique_name"`)
+	// Type-assert returned error.
+	var pqerr *pq.Error
+	require.True(t, errors.As(err, &pqerr))
+	require.Equal(t, `duplicate key value violates unique constraint "unique_name"`, pqerr.Message)
+	require.Equal(t, user.Table, pqerr.Table)
+	require.Equal(t, "unique_name", pqerr.Constraint)
+	require.Equal(t, pq.ErrorCode("23505"), pqerr.Code, "unique violation")
+}